The long-term goal of this project is to determine how RNA polymerase I (pol I) transcription is linked to the signals that control cell growth. This is relevant to human diseases such as cancer that involve altered gene expression and disrupted signal transduction pathways. Ribosomal RNA (rRNA) transcription provides an excellent model system in which to study growth regulated gene expression because pol I activity is coordinated with the growth status of the cell. This regulation is crucial because rRNA levels directly affect ribosome production and the cell's capacity to make proteins. The same signalling networks that control polymerase I activity are apparently deregulated in cancer cells. Normal cells are generally dependent on serum for growth and stop growing when they contact other cells. Likewise, pol I transcription in normal cells is down- regulated upon serum starvation or contact inhibition. However, serum independence and loss of contact inhibition are common characteristics of tumor cells, as are prominent nucleoli (where pol I transcription takes place). Determining the biochemical basis for growth regulation of RNA polymerase I is important for two major reasons. First, it will provide basic information about the mechanisms that transmit signals between the cytoplasm and nucleus to modulate RNA polymerase activity in eukaryotic cells. Second, the knowledge gained may facilitate the search for drugs that can slow the growth of tumor cells by blocking RNA pol I activation. Evidence suggests that pol I transcription is regulated by modification of one or more key transcription factors or the polymerase itself. However, the means by which control is exerted are unclear. Two major hypotheses are consistent with available data. One is that modification of one or more transcription factors controls assembly of the preinitiation complex. An alternative hypothesis is that modification of one or more key activator proteins affects RNA polymerase initiation or elongation after transcription complex assembly. The proposed research will address these hypotheses using fractionated transcription extracts and purified transcription factor complexes from cultured Xenopus laevis cells coupled with a gel mobility shift assay to visualize transcription complex assembly. These studies will develop the methods to identify control points in growth-regulated rRNA gene transcription, a first step in the identification of signalling pathways and molecules that exert this control.